When an optical device having a waveguide is used for controlling propagating light or for emitting light externally, a spot shape of light is required to be a single-peaked shape in view of convenience.
When two waveguides having different waveguide modes are optically coupled, it is necessary to couple the two waveguides under a state in which the single-peaked shape is maintained at high efficiency. However, when the two waveguides having different waveguide modes are directly butt-coupled, coupling efficiency is deteriorated so that the loss increases. In addition, in the coupled area, the waveguide mode is apt to be disturbed, and the light spot shape does not become the single-peaked shape.
As a conventional technology for dealing with those problems, there is known a method involving using a mode converter having a tapered structure or a stepped waveguide as disclosed in Japanese Patent No. 2850996 or Japanese Patent Application Laid-Open No. H08-171020.
FIGS. 12A and 12B illustrate mode converters having the tapered structure and the stepped waveguide according to those conventional examples. FIG. 12A illustrates a mode converter 400 having the tapered structure, and FIG. 12B illustrates a mode converter 401 having the stepped waveguide.
The mode converter having the tapered structure or the stepped waveguide illustrated in FIGS. 12A and 12B has a structure in which the width of a core part 106 (the length in the x direction in FIGS. 12A and 12B) is gradually changed so that the light spot size can be changed gradually while maintaining the single-peaked shape of the propagating light. Thus, the optical coupling is made, or the light is emitted externally.